Real-time computer image systems are designed to provide realistic image reproduction for a wide variety of graphics systems, such as tank/vehicle or flight simulators, action games and engineering work stations. For example, graphics-based simulators are used as training devices which permit a combatant to obtain practice without the necessity of going out into the field and using the actual combat systems. The simulators enable a user, such as a pilot or tank gunner, to maintain and improve his skills without the cost associated with live training. It is thus very advantageous to provide the user with video display realism which is beneficial for training and practice purposes.
In a typical real-time computer image generation system, such as a simulator system, image generation can be broken into three separate processing stages: Controller, Geometry Processor, and Display Processor. These three processing stages or sections each work independently on data representative of or corresponding to one of three consecutive scenes to be displayed. The Controller processes data on a scene or image for a fixed time, which may be either a field time of 16.67 milliseconds or a frame time of 33.3 milliseconds, usually the former, and then passes its processed data to the Geometry Processor. This particular field and frame time corresponds to that time for a typical television format display, i.e., two interlaced fields per frame with thirty frames per second. The Geometry Processor has an equal time to do its calculations and at the end of that time its results are sent to the Display Processor. The Display Processor always processes one interlace field for a video display in one field time. For each time interval, all three processors are kept busy, so that, at any given time, the processors are working on three separate scenes. Each section processes inputs and generates outputs during the interval, so data flows through the three stages in a sequential manner. The computational load is spread out among the specialized processor sections, so this allows new scenes to be displayed each field or frame time, even though the results are delayed three fields or frames.
For example, in a flight simulator system, pilot inputs are received by the Controller and after being processed sequentially by the Controller, the Geometry Processor and Display Processor, the output is sent as a color scene to video displays observed by the pilot in the simulator. The data seen by the pilot starts out as simple control inputs applied to an environment definition and is converted to pixel video display information. In the controller, the pilot inputs are used to calculate the new position and orientation of the aircraft, and from this, a mathematical rotation matrix is calculated that is used to rotate objects from the reference coordinate system to a display coordinate system. Any other information dealing with the aircraft, with the position of targets and with other aircraft is also calculated in the Controller. In some applications, the Controller is a general-purpose computer.
The Geometry Processor reads, from a database, descriptions of objects that are potentially visible in the stored three-dimensional digital representation of the scene. The objects that are read are rotated into display coordinates using the rotation matrices calculated in the Controller. Edges of these three-dimensional objects are clipped by the Geometry Processor if they extend beyond the view window boundaries. The Geometry Processor mathematically projects the three-dimensional data onto the two-dimensional display window. In addition, the Geometry Processor calculates which objects are in front or behind other objects and stores this information in a priority list. Each object processed is made up of individual polygons bounded by straight edges. The priority list contains the order of all polygons in the scene, with the first polygon in the list as the highest priority polygon, and the last polygon in the list has the lowest priority. Whenever two polygons overlap on the display, the higher priority polygon will be visible, and the lower priority polygon will be obscured.
The Display Processor receives the two-dimensional face descriptions, along with face color, face fading information, a face priority list, cell texturing, level-of-detail blending, translucency, curvature shading, etc., and uses this information to output data describing to a color display each pixel in the scene. Each color display in the system is typically made up of 1024 lines of video information, with each line having 1024 individual color dots or pixels. However, other combinations of lines and pixels per line are certainly possible. The Display Processor has to resolve, for each pixel, what the color and luminous intensity of the pixel should be, given that many faces can be present in a pixel, that certain portions of faces may be covered by other faces, and that several light sources may be illuminating each surface portion reflecting light into that pixel. Since there can be 1024 by 1024 pixels in the display, and all these need to be calculated in 1/60th of a second, the processing load in the Display Processor is very high. To calculate video for a multi-display system (a single system has contained as many as 14 displays), the Display Processor must perform in excess of 10 billion computations per second. Due to the tremendous processing load, a Display Processor can drive only a few displays. In some systems, a single Controller and Geometry Processor can be used to drive several Display Processors with several displays connected to each Display Processor. The Display Processor therefore represents the majority of processing in a real-time computer image generation system, and for this reason most of the effort to improve the capabilities and realism of a CIG system is concentrated in the Display Processor area.
One class of problems has been to process plural sources of illumination, in any one scene, so as to not only prevent saturation of pixel color intensity information, but also to provide shadowing of any visible polygon surface as necessary by each of the several light sources, and to do so in a low-cost, efficient manner which also prevents aliasing of information.